Introduction

Most species have large, muscular females, ca. 1 m or more in total length, that occupy surface waters of tropical and subtropical oceans. In females the dorsal and dorsolateral arms are distinctly longer than arms III and IV and are connected by an extensive web which is absent from the other arms.

The color phase most typically associated with T. violaceus is with silvery sides and a very dark purple/blue dorsal surface.

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Figure. The picture of T. violaceus on the left, taken in an aquarium, shows the typical color pattern. Photograph taken by Vicente Hernandez. The photograph on the right of Tremoctopus sp. taken from a submersible shows the octopod swimming at a depth of 340 m and about 5 m above the bottom in Hawaiian waters. An AVI format video clip of this individual is available at Cephalopods in Action. Submersible photograph courtesy of the Hawaii Undersea Research Laboratory.

Brief diagnosis:

An argonautoid ...

with extensive web between dorsal four arms; web virtually absent between other arms (females).

Characteristics

Arms

Dorsal four arms (arms I and II) much longer than ventral four arms (arms III and IV).

Deep web present between dorsal four arms (see title photograph).

Hectocotylus develops in sac burried beneath right eye.

Proximal half of hectocotylus with papillate lateral fringes.

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Figure. Hectocotylus of Tremoctopus violaceus.Top - Oral view of the hectocotylus still attached to the male. Bottom - Median section through the distal end of the hectocotylus with an empty spermatophore receptacle. Drawings from Naef, 1921-3.

The systematics of the Tremoctopodidae was review by Thomas (1977). He concluded that two species existed, T. violaceus and his new species T. gelatus Thomas, 1977. The former species he divided into two subspecies, T. v. violaceus, from the Atlantic Ocean and T. v. gracilis, from the Pacific and Indian Oceans. O'Shea (1999) has resurrected a New Zealand species, T. robsoni. Since the degree of difference between the latter and T. violaceus is of the same order as that between the two subspecies of T. violaceus we here elevate the two subspecies to specific level:

There is some confusion on the proper name of T. robsoni. Mike Sweeney (USNMNH) explains: "The name Tremoctopus robsonianus is in the title of the paper (an abstract from a meeting of the Wellington Philosophical Society). However the taxon is introduced in the text with the name Tremoctopus robsoni n.sp. The taxon was published in the 1883 volume (#16) of the Transactions of the New Zealand Institute, but had a publication date of May 1884." The correct name is Tremoctopus robsoni Kirk, 1884.

Species

Of the four recognized species, T. gelatus is easily recognized by its gelatinous consistency and generally pale pigmentation, the other three species are very similar. The most useful character is the number of sucker pairs on the distal half of the hectocotylus of the male. Another character is the number of gill filaments on the outer demibranch of the gill not including the terminal filament.

A few additional features on the hectocotylus seem to have specific value. The proximal suckers number 22-23 pairs in T. violaceus, 27-29 pairs in T. gracilis (Thomas, 1977) and 9-15 in T. robsoni according to the figures shown here from O'Shea, 1999. Also the hectocotylus of the first two species has a fringe of fleshy papillae that extend the full length of the proximal section, while the third species, according to the illustration, has the fringe restricted to the base of the proximal section.

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Figure. Oral views of three hectocotyli of T. robsoni. Drawings from O'Shea, 1999.

Behavior

Young individuals carry broken tentacles of cnidarians, frequently the Portuguese man-of-war (jellyfish), using the suckers of the dorsal four arms. The borrowed tentacles, which have stinging cells, presumably have a defensive and/or offensive function.

Life History

Males are dwarfs (15 mm ML), often reaching only 5-10% of the female size. The females carry numerous (100,000 to 150,000) small eggs (0.9 X 1.5 mm in size). The eggs are attached to a secreted sausage-shaped rod held at the base of the dorsal arms and carried by the female until hatching. The hatchling has the arm bases in a cuff as in Argonauta. Data are from Thomas (1977) and Naef (1921/23). More information of the brooding female, the egg mass, embryos and hatchlings can be found here.

Distribution

T. violaceus lives in the Atlantic and T. gracilis in the Indo-Pacific; T. gelatus is a deep-living, gelatinous, presumably mesopelagic, species that is cosmopolitan in tropical and temperate seas (Thomas, 1977). T. robsoni is known from waters off New Zealand (O'Shea, 1999).

Page: Tree of Life
Tremoctopodidae Tryon, 1879. TremoctopusChiaie 1830. Blanket octopus.
Authored by
Katharina M. Mangold (1922-2003), Michael Vecchione, and Richard E. Young.
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